Rechargeable prismatic batteries are used in a variety of industrial and commercial applications such as fork lifts, golf carts, uninterruptable power supplies, and electric vehicles.
Rechargeable lead-acid batteries are presently the most widely used type of battery. Lead-acid batteries are a useful power source for starter motors for internal combustion engines. However, their low energy density, about 30 wh/kg makes them an impractical power source for an electric vehicle. An electric vehicle using lead acid batteries has a range of only about 30 to 120 miles before requiring recharge. Lead acid batteries require about 6 to 12 hours to recharge and contain large amount of toxic materials. In addition, electric vehicles using lead-acid batteries have sluggish acceleration, top speeds of only 50 to 60 mph, and a battery lifetime of only about 20,000 miles.
Nickel metal hydride batteries ("Ni-MH batteries") are far superior to lead acid batteries, and Ni-MH prismatic batteries are the most promising type of battery available for electric vehicles. For example, Ni-MH batteries, such as those described in U.S. patent application No. 07/934,976 now U.S. Pat. No. 5,277,999 to Ovshinsky and Fetcenko, the disclosure of which is incorporated herein by reference, have a much better energy density than lead acid batteries: they can power an electric vehicle over 250 miles before requiring recharge, they can be recharged in less than one hour, and they contain no toxic materials. Electric vehicles using Ni-MH batteries will have an acceleration of 0-60 in 8 seconds, a top speed of 100 mph, and a battery lifetime of more than about 100,000 miles.
Most Ni-MH batteries use nickel hydroxide positive electrodes and hydrogen storage negative electrodes. The electrodes are separated by non-woven, felted, nylon or polypropylene separators. The electrolyte is usually an alkaline electrolyte, for example, containing 20 to 45 weight percent potassium hydroxide.
Ni-MH batteries were previously classified based on whether they used AB.sub.2 or AB.sub.5 alloys as the hydrogen storage material of the negative electrode. The distinction between AB.sub.2 or AB.sub.5 alloys has disappeared as they have evolved. Both types of material are discussed in detail in U.S. Pat. Nos. 5,096,667 and 5,104,617 and U.S. patent application Nos 07/746,015 (now U.S. Pat. No. 5,238,756) and 07/934,976. The contents of all these references are specifically incorporated by reference. AB.sub.2 alloys are now commonly referred to as Ovonic alloys.
Simply stated, in the AB.sub.5 alloys, like the Ovonic alloys, as the degree of modification increases, the role of the initially ordered base alloy is of minor impodance compared to the properties and disorder attributable to the particular modifiers. In addition, analysis of the present multiple component AB.sub.5 alloys indicates that these alloys are modified following the guidelines established for Ovonic alloy systems. Thus, highly modified AB.sub.5 alloys are the same as Ovonic alloys. Both Ovonic alloys and AB.sub.5 alloys are disordered materials characterized by multiple components and multiple phases. Both are thus multicomponent and multiphase alloys between which there no longer exists any significant distinction.
In electric vehicles, the weight of the batteries is a significant factor because battery weight is the largest component of the weight of the vehicle. For this reason, reducing the weight of individual batteries is a significant consideration in designing batteries for electric powered vehicles. In addition to reducing the weight of the batteries, the reliability of the specific components of the battery need to be improved. One particular area in need of improvement is the electrode-terminal-external connector area.
Most prismatic batteries presently in use are vented batteries that operate at around 16 psi and require constant maintenance. In contrast, Ni-MH prismatic batteries using Ovonic alloys are designed as a sealed, maintenance free system. These batteries operate at around 100 psi.
Presently, in prismatic electric vehicle batteries, the battery terminals are solid, cast, cylindrically shaped and formed from copper or a copper alloy. The terminals are routinely threaded male ends that are screwed into the battery case lid. To insure that the terminal forms the required pressure seal between the case lid and the terminal, an O-ring seal is commonly placed between two plastic washers and this combination placed between the terminal and the case lid. The tabs of the battery's electrodes are then bolted to the bottom of the terminals, thereby making the required electrical connections. The external electrical connection to other batteries or to the final battery leads is made by bolting the connectors to the terminals.
Terminals in prismatic batteries as they presently exist are heavy and expensive because they are solid cast metal, threaded into the lid. It would be difficult to significantly reduce the weight of such terminals without effecting their structural integrity. Also, the terminals have a tendency to loosen when used in an environment in which the batteries are subjected to physical vibration, such as in electric vehicles. Loosening can result in unwanted venting of cell gases and shorting of the terminal to the battery case.
Electrode tabs (i.e. the internal electrical connectors between the battery electrode plates and the battery terminals) are gathered together and physically bolted to their respective battery terminals. The space required for bolting necessitates a minimum required overhead space (head space) between the tops of the battery electrode plates and the top of the battery case. This requires a length of battery case and a length of electrode tab material both of which add to the weight of the battery without a corresponding increase in battery storage capacity. An additional drawback is that vibrational loosening of the bolts which connect the electrodes to the terminals can cause high resistance pathways, reducing performance.
Present batteries are also expensive to manufacture. This is due to the number of parts they contain, as well as the time involved in assembling the component parts into the final product. Finally, heat generated during charge and discharge can become significant in Electric Vehicle (EV) batteries because it increases the degradation of electrodes, separators, and electrolytes, thereby reducing the life of the battery.
Thus, there exists the need for a battery design in which the electric connection components are resistant to the negative effects of a high vibration environment in a manner that reduces the overall weight of the battery without reducing its energy storage capacity, increases the batteries' reliability, and decreases the cost.